When Math meets Android Animation (1)

/**
 * A time interpolator defines the rate of change of an animation. This allows animations
 * to have non-linear motion, such as acceleration and deceleration.
 */
public interface TimeInterpolator {
    /**
     * Maps a value representing the elapsed fraction of an animation to a value that represents
     * the interpolated fraction. This interpolated value is then multiplied by the change in
     * value of an animation to derive the animated value at the current elapsed animation time.
     */
    float getInterpolation(float input);
}

/**
 * Interface for use with the {@link ValueAnimator#setEvaluator(TypeEvaluator)} function. Evaluators
 * allow developers to create animations on arbitrary property types, by allowing them to supply
 * custom evaluators for types that are not automatically understood and used by the animation
 * system.
 */
public interface TypeEvaluator<T> {

    /**
     * This function returns the result of linearly interpolating the start and end values, with
     * <code>fraction</code> representing the proportion between the start and end values. The
     * calculation is a simple parametric calculation: <code>result = x0 + t * (x1 - x0)</code>,
     * where <code>x0</code> is <code>startValue</code>, <code>x1</code> is <code>endValue</code>,
     * and <code>t</code> is <code>fraction</code>.
     */
    public T evaluate(float fraction, T startValue, T endValue);
}

/**
 * This method is called with the elapsed fraction of the animation during every
 * animation frame. This function turns the elapsed fraction into an interpolated fraction
 * and then into an animated value (from the evaluator. The function is called mostly during
 * animation updates, but it is also called when the <code>end()</code>
 * function is called, to set the final value on the property.
 */
void animateValue(float fraction) {
    fraction = mInterpolator.getInterpolation(fraction); //TimeInterpolator 函数
    mCurrentFraction = fraction;
    int numValues = mValues.length;
    for (int i = 0; i < numValues; ++i) {
        mValues[i].calculateValue(fraction); //TypeEvaluator 函数
    }
    if (mUpdateListeners != null) { // 通知监听器
        int numListeners = mUpdateListeners.size();
        for (int i = 0; i < numListeners; ++i) {
            mUpdateListeners.get(i).onAnimationUpdate(this);
        }
    }
}

//默认的TimeInterpolator是AccelerateDecelerateInterpolator
// The time interpolator to be used if none is set on the animation
private static final TimeInterpolator sDefaultInterpolator = new AccelerateDecelerateInterpolator();

/**
* The time interpolator used in calculating the elapsed fraction of this animation. The
* interpolator determines whether the animation runs with linear or non-linear motion,
* such as acceleration and deceleration. The default value is
* {@link android.view.animation.AccelerateDecelerateInterpolator}
*
* @param value the interpolator to be used by this animation. A value of <code>null</code>
* will result in linear interpolation.
*/
@Override
public void setInterpolator(TimeInterpolator value) {
   if (value != null) {
       mInterpolator = value;
   } else {
       // 如果传入的TimeInterpolator是null的话就设置为LinearInterpolator
       mInterpolator = new LinearInterpolator();
   }
}

/**
 * The type evaluator to be used when calculating the animated values of this animation.
 * The system will automatically assign a float or int evaluator based on the type
 * of <code>startValue</code> and <code>endValue</code> in the constructor. But if these values
 * are not one of these primitive types, or if different evaluation is desired (such as is
 * necessary with int values that represent colors), a custom evaluator needs to be assigned.
 * For example, when running an animation on color values, the {@link ArgbEvaluator}
 * should be used to get correct RGB color interpolation.
 *
 * <p>If this ValueAnimator has only one set of values being animated between, this evaluator
 * will be used for that set. If there are several sets of values being animated, which is
 * the case if PropertyValuesHolder objects were set on the ValueAnimator, then the evaluator
 * is assigned just to the first PropertyValuesHolder object.</p>
 *
 * @param value the evaluator to be used this animation
 */
public void setEvaluator(TypeEvaluator value) {
    if (value != null && mValues != null && mValues.length > 0) {
        mValues[0].setEvaluator(value);
    }
}

//根据提供的数字序列得到动画的核心帧集合
public static KeyframeSet ofInt(int... values) {
    int numKeyframes = values.length;//有多少个数字就有多少帧
    IntKeyframe keyframes[] = new IntKeyframe[Math.max(numKeyframes,2)];//至少有2帧
    if (numKeyframes == 1) {//如果只传入一个数字，那么该数字就是结束帧的值
        keyframes[0] = (IntKeyframe) Keyframe.ofInt(0f);
        keyframes[1] = (IntKeyframe) Keyframe.ofInt(1f, values[0]);
    } else {//如果传入多个数字，那么可以将整个动画时间间隔均分，每个数字按顺序在每个时间比率上占据一个属性值
        keyframes[0] = (IntKeyframe) Keyframe.ofInt(0f, values[0]);
        for (int i = 1; i < numKeyframes; ++i) {
            keyframes[i] = (IntKeyframe) Keyframe.ofInt((float) i / (numKeyframes - 1), values[i]);
        }
    }
    return new IntKeyframeSet(keyframes);
}

/**
 * Gets the animated value, given the elapsed fraction of the animation (interpolated by the
 * animation's interpolator) and the evaluator used to calculate in-between values. This
 * function maps the input fraction to the appropriate keyframe interval and a fraction
 * between them and returns the interpolated value. Note that the input fraction may fall
 * outside the [0-1] bounds, if the animation's interpolator made that happen (e.g., a
 * spring interpolation that might send the fraction past 1.0). We handle this situation by
 * just using the two keyframes at the appropriate end when the value is outside those bounds.
 */
public Object getValue(float fraction) {
    // Special-case optimization for the common case of only two keyframes
    if (mNumKeyframes == 2) {//1.处理只有2帧的情况
        if (mInterpolator != null) {
            //先调用TimeInterpolator函数
            fraction = mInterpolator.getInterpolation(fraction);
        }
        //再调用TypeEvaluator函数
        return mEvaluator.evaluate(fraction, mFirstKeyframe.getValue(), mLastKeyframe.getValue());
    }
    //2.处理上冲和下冲的情况
    if (fraction <= 0f) {
        final Keyframe nextKeyframe = mKeyframes.get(1);
        final TimeInterpolator interpolator = nextKeyframe.getInterpolator();
        if (interpolator != null) {
            fraction = interpolator.getInterpolation(fraction);
        }
        final float prevFraction = mFirstKeyframe.getFraction();
        float intervalFraction = (fraction - prevFraction) / (nextKeyframe.getFraction() - prevFraction);
        return mEvaluator.evaluate(intervalFraction, mFirstKeyframe.getValue(), nextKeyframe.getValue());
    } else if (fraction >= 1f) {
        final Keyframe prevKeyframe = mKeyframes.get(mNumKeyframes - 2);
        final TimeInterpolator interpolator = mLastKeyframe.getInterpolator();
        if (interpolator != null) {
            fraction = interpolator.getInterpolation(fraction);
        }
        final float prevFraction = prevKeyframe.getFraction();
        float intervalFraction = (fraction - prevFraction) / (mLastKeyframe.getFraction() - prevFraction);
        return mEvaluator.evaluate(intervalFraction, prevKeyframe.getValue(), mLastKeyframe.getValue());
    }
    //3.处理正常的多帧的情况
    Keyframe prevKeyframe = mFirstKeyframe;
    //首先要遍历前面计算出的主要KeyFrame集合，看当前的fraction是处在哪个区间的
    for (int i = 1; i < mNumKeyframes; ++i) {
        Keyframe nextKeyframe = mKeyframes.get(i);
        if (fraction < nextKeyframe.getFraction()) {
            final TimeInterpolator interpolator = nextKeyframe.getInterpolator();
            final float prevFraction = prevKeyframe.getFraction();
            //将当前的fraction折算成在这个区间内的时间比率，这个计算有意思吧
            float intervalFraction = (fraction - prevFraction) / (nextKeyframe.getFraction() - prevFraction);
            // Apply interpolator on the proportional duration.
            if (interpolator != null) {
                //先调用TimeInterpolator函数
                intervalFraction = interpolator.getInterpolation(intervalFraction);
            }
            //再调用TypeEvaluator函数
            return mEvaluator.evaluate(intervalFraction, prevKeyframe.getValue(), nextKeyframe.getValue());
        }
        prevKeyframe = nextKeyframe;
    }
    // shouldn't reach here
    return mLastKeyframe.getValue();
}

public Integer evaluate(float fraction, Integer startValue, Integer endValue) {
    int startInt = startValue;
    return (int)(startInt + fraction * (endValue - startInt));
}